By all reports, Dempsey was hard on sparring partners, so it may have been a tad bit more than "just a sparring session." Probably that's why it didn't last that long and only happened once. They both had reputations to uphold, after all.
Judging Greb by the O'Brien footage is like gaging Tunney by the Corbett footage .. to accurately judge Greb take the best footage of Tunney from Dempsey/Heeney/Carpentier and Gibbons and know that this middleweight gave him the toughest fights of his career ...